The question whether DNA is electrically conducting has
attracted much interest lately. Experiments have turned up a distressingly
large range of values for the equivalent resistivity r (from 10-4 Ohm cm to 106 Ohm-cm).
Proximity induced superconductivity has also been reported.

We have approached this problem by forming direct chemical
bonds between the ends of lambda-DNA to Au electrodes. The idea is to
insert, at both ends of the DNA molecule, thyosines (T) that have been modified
to include the thiol group -HS. The thiol group -HS has a high propensity
to bind to Au. This provides the electrical contact between DNA and our
Au electrodes. The top figure shows a cartoon of one of the original
sticky ends of lambda-DNA which is comprised of 48,500 base pairs occupying a
double helix 16 microns in length (see top right for color code).

In the middle figure, the DNA molecule is immersed in a solution of mostly
modified thyosine (T') and lesser amounts of guanine (G) and adenine (A)
[cytosine (C) is left out]. The DNA polymerase (large blue blob) proceeds
to incorporate the modified T's, G and A nucleotides into both sticky
ends. The absence of cytosines in solution means that the modified T's
are paired with G's. Normally, such mismatches are rapidly deleted when
the polymerase proof-reads its own actions. To suppress this undesired
editing, we use a mutated polymerase that lacks this proof-reading
capability. The bottom figure shows the sticky end filled-in with excess
modified T's.

A drop of the solution of lambda-DNA (with ends modified) is
placed on a substrate containing gold electrodes (light vertical stripes in
figure). The DNA molecules are rendered visible by binding them to dye
molecules. The 4 panels on the left (A1...B2) show DNA molecules
'flexing' in an alternating solution flow (flow indicated by arrows). The
binding energy of the ends is sufficient to resist the strong shear forces in a
strong flow. The right panel shows scores of DNA molecules with ends
bound to different gold electrodes.

For electrical measurements, the solution is evaporated in a vacuum and a
voltage is applied (up to 20 volts). These experiments show that
lambda-DNA has a resistivity larger than 106 Ohm cm at room
temperature. Our results contradict many reports of moderately high
conductivity in lambda-DNA.